The present disclosure relates generally to suspension components on vehicles. More particularly, the present disclosure relates to a shock absorber with a gas spring seal arrangement with reduced friction during compression for use on bicycles.
Reducing friction in the compression stroke may improve suspension response and allow for finer control of compression damping. These improvements are of particular interest for off-road cycling, where the combination of light component weight and suspension compliance is highly valued.
Shock absorbers that support the weight of the vehicle with compressed gas instead of coil or leaf springs may be attractive for applications where the weight of components must be kept as low as possible. Moreover, gas spring shocks may allow for convenient adjustability of the spring rate of the suspension, in some cases by increasing or decreasing the volume of gas within the shock. Both of these advantages have made gas spring shock absorbers a popular choice for mountain bikes. However, precisely because the ratio of vehicle-to-passenger weight may be low for bicycles, jounce may be transmitted efficiently and may be felt keenly by the cyclist. Where terrain is rugged, as in off-road cycling, it may be desirable for the bicycle's suspension to be as responsive as possible. The difference in responsiveness between gas-sprung and coil-sprung shocks has proven great enough to limit the use of gas-sprung shocks in off-road cycling.
Turning to FIGS. 1-4, a conventional gas spring shock may be seen. A conventional gas spring shock absorber 10 for lightweight vehicles, including bicycles, employs a piston 16 with a gas seal 35 that engages a cylinder 12. As the seal 35 on the piston 16 moves against the wall of the cylinder 12 during compression, the gas trapped in a compression chamber 22 between the seal 35 and the closed end 32 of the cylinder 12 offers progressively greater resistance to compressive movement as a simple function of rising pressure against the sectional area of the piston 16 and seal 35. Secondarily, this rising gas pressure causes the piston seal 35 to press with progressively greater force against the cylinder 12. The frictional adhesion of the seal 35 to the cylinder wall must be overcome before the shock absorber 10 will compress, decreasing responsiveness.
If an o-ring seal (not shown) is used on the piston of a conventional gas spring design, the contact area of the seal with the cylinder surface will be relatively large. The area of o-ring contact varies with pressure in the gas spring chamber, since pressure forces the o-ring axially toward one end of its gland and into conformity with the square-cornered sectional profile formed by the gland and cylinder wall. The relatively large contact patch of the o-ring under pressure adds significantly to the adhesion of the seal to the cylinder.
A u-cup or “X”-section seal (as shown in FIGS. 1-4) will have a smaller area of contact with the cylinder compared to an o-ring. However, in order to contain gas pressure reliably, the projecting lip 35a of such a seal must be angled toward the cylinder in the direction of compression and in the direction of its own movement in relation to the dynamically sealed surface. The angular abutment of the seal lip against the cylinder wall 12 drives the pressure of the seal material at the contact patch to high values during compression. The chisel-action of the seal lip creates significant adhesion of the seal material to the cylinder wall, partially offsetting the advantage of a smaller area of contact as compared with an o-ring.
The need therefore exists for a gas spring shock absorber with reduced adhesion of the dynamic gas containment seal during the compression stroke, thereby improving responsiveness.